The benefit of seat belts in preventing or limiting injury during a motor vehicle crash is substantially enhanced if a system to detect the onset of a crash is used in combination with a device known as a seat belt retractor which tightens the seat belt in response to the detection of a crash of a selected severity. The reeling of the belt onto the belt spool is termed “retraction,” while the unwinding of belt is known as “protraction.” Further benefit is derived from using a seat belt spool which incorporates a load limiting torsion rod, the load limiting torsion rod allows seat belt webbing to be drawn from the seat belt spool under load to absorb energy from a vehicle occupant restrained by the seat belt. The load limiting mechanism is brought into operation by a level of load applied to the belt, or the rate at which the seat belt is protracted. Typically, the load limiting mechanisms operate by locking the normal mechanisms that allow the seat belt to be protracted from a seat belt take-up spool. Locking of the protraction mechanism where large loads are applied to the seat belt results in shearing a shaft which connects the seat belt spool to a seat belt bracket which is in turn anchored to the structure of the vehicle, or a vehicle seat. Seat belts that incorporate retractors and load limiters are used in combination with airbags to further prevent or limit injury during a motor vehicle crash. Ideally the seat belt restrains the occupant during the initial part of the crash until the airbags are deployed.
After airbag deployment, the vehicle occupants engage one or more airbags and the restraining effect of the seat belts becomes complementary to the restraining effect of the airbags. Load limiting mechanisms typically take the form of a shaft which is allowed to plastically deform under a torsional shear load such as the system described in U.S. Pat. No. 6,0012,667 which is incorporated herein by reference. Other types of energy absorbing mechanisms are also known and generally use the plastic deformation of a structure to absorb energy.
In a less severe crash, the airbags typically will not deploy, as deployment of airbags means the airbags must be replaced at some considerable expense and there is some risk associated with airbag deployment. The safety system in the motor vehicle determines whether or not to deploy an airbag or other safety system feature operates according to various algorithms based on sensor inputs to determine as well as possible whether airbag deployment would be beneficial or not. If an airbag is not deployed, generally no maintenance is required, and the unit does not have to be replaced. On the other hand, if the airbag has deployed it is in obvious need of replacement.
Similar to airbags deployment, seat belt retractor deployment is controlled by the safety system in the motor vehicle and normally results in the actuation of a pyrotechnic mechanism the operation of which is more or less apparent. However, the seat belt spool on which the seat belt webbing is wound normally locks in response to a relatively low level of deceleration that may occur on an almost daily basis for a typical motor vehicle. However, once the seat belt spool is locked, significant further payout of the seat belt results in shearing of the load limiting torsion rod. After a crash, the vehicle is normally repaired and the airbags, if deployed, are replaced, but the need for replacing the load limiting torsion rods which may have undergone plastic yielding is less obvious as the seat belt and the seat belt spool and the retracting mechanisms may remain functional.
What is needed is a method and instrumentation for monitoring the operation and the loads experienced by the load limiting torsion rod, so that the safety system can determine whether the load limiting torsion rods need to be replaced and inform the vehicle operator when a need for replacement has been identified. Monitoring the operation of the load limiting torsion rod would also supply additional data to the safety system which could be considered by the safety system logic in determining the timing and force with which other safety systems such as airbags are deployed.
A further function which such a sensor could provide would be to provide data about whether a passenger is properly belted. Airbag deployment strategies can be varied depending on whether or not a passenger is restrained by seat belts. However, using such a deployment strategy requires a way of detecting that the seat belt is not only fastened, but also fastened about the passenger. If the seat belt is fastened but not about the occupant the motor vehicle safety system may not respond correctly unless a way can be found to determine that although the seat belt is fastened it is not operating to restrain an occupant.